Potential Impacts of Zebra Mussel Invasion on Gamefish Health and Prey Availability in Prairie Lakes

Abstract

Invasive species are a threat to global aquatic ecosystems. Nonetheless, their impacts can be heterogeneous across systems. Recent reports suggest Saskatchewan (SK) prairie lakes are highly prone to zebra and quagga mussels (ZQM) given their proximity to invasive corridors of Manitoba and three US states (Montana, North Dakota, and Minnesota) where they have been reported. As the effect of ZQM on native species is variable and context-dependent across locations and time, an understanding of prevailing biological, physical and chemical conditions supporting various cohorts of fish communities is necessary for prioritizing mitigation efforts to address the potential effects of these dreissenids in uninvaded systems. To gain these important insights, I investigated diet composition and body conditions of three ecologically and recreationally important gamefishes; walleye (Sander vitreus), northern pike (Esox lucius), and yellow perch (Perca flavescens) sampled from 18 SK prairie lakes. In addition, I assessed the impact of fish size and environmental variables on gamefish body conditions. Twenty-three prey taxa were found in the diets of these fishes. While both invertebrates (amphipods) and fish formed significant constituents of walleye diets, pike concentrated more on fish prey, whereas perch relied more on invertebrates (chironomids and amphipods). Zooplankton was only a minor contributor to the diets of all three gamefish species. Body conditions of walleye and pike decreased with size, but yellow perch populations were in consistently good condition across all size groups. These results suggest preferred fish prey of larger piscivores (i.e., pike and walleye) may be present in insufficient quantity. Lake productivity and alkalinity influenced body conditions of large-sized walleye and yellow perch. The scarcity of pelagic zooplankton prey in the diets of small-sized gamefishes implies these populations may be less threatened by future ZQM invasion than previously assumed. Next, I contrasted stomach content data and Carbon (C), Nitrogen (N), with Sulfur (S) stable isotope analyses to better quantify the relative importance of various foraging habitats to the three gamefish species. Bayesian Mixing models predicted that in about 70% of our study lakes > 55% of gamefish diets were sourced from the littoral region, whereas in ~30% of surveyed lakes, > 55% of gamefishes diets was sourced from the pelagic habitat. General evidence of similar reliance on sediment S sources in lakes where two species co-occurred (walleye and pike or walleye and perch) was observed. Overall, the three predominant gamefishes exhibited comparable dependence on the three feeding habitats (littoral, pelagic, sediment) across SK prairie lakes. Findings of stomach content analysis (SCA) and stable isotope analysis (SIA) were assessed to determine how the two techniques corroborate, contradict or complement each other. Results indicated a lack of correspondence between the two methods utilized in examining walleye diets, suggesting the need to use both techniques as complements for more robust results. Finally, because future invasion by ZQM is expected to significantly impact energy sources of gamefishes, I examined the current environmental controls of reliance on pelagic, littoral, and sediment-based diets. Walleye reliance on pelagic energy sources increased with the maximum depth of lakes and decreased with total dissolved solids (TDS) concentrations, implying the contribution of pelagic diets increase for fish populations in deeper and less saline lakes. However, sediment-based diets were not influenced by any of the selected environmental parameters. Integrating SCA and SIA overall helped to advance our understanding of feeding habits of gamefish populations in SK prairie lakes and demonstrates the value of combining these analyses for a comprehensive understanding of feeding interactions within aquatic ecosystems.